Proper development of functional gametes (sperm and eggs) is crucial for the continuation of sexually reproducing species. In order to succeed, these highly differentiated cells must fuse and form a totipotent zygote capable of developing into a complex organism. Furthermore, gametes are responsible for maintaining genomic integrity from generation to generation. Thus far, genetic screens mice and popular ecdysozoan model organisms, such as Drosophila melanogaster and Caenorhabditis elegans, have identified numerous genes required for germline development and function. However, voids remain to be filled in our understanding of the mechanisms that secure the production of functional gametes. In order to fill some of these voids, the proposed line of research will provide a new angle for discovery of genes with conserved roles in mammalian germline development. The planarian flatworm Schmidtea mediterranea is a greatly suitable yet under-utilized organism that holds promise as a research model in the field of reproductive biology due to its unmatched ability to completely regenerate its entire reproductive system throughout its lifetime. This remarkable regenerative ability allows for analysis of the entire process of gonad and germline development without interfering with embryonic development or being limited to a specific developmental window. Also important is the fact that like mammals, but unlike flies or nematodes, planarians specify their germline via inductive signals that originate from the soma. Finally, planarians hold a unique evolutionary relationship to mammals when compared to ecdysozoans, which results in conservation of genes not present in nematodes or flies. The experiments performed as part of this proposal are expected to reveal novel conserved pathways that control reproductive biology by: 1) Elucidating the transcriptome of female and male reproductive organs in S. mediterranea hermaphrodites; 2) Discovering genes required at different stages of oogenesis and spermatogenesis, as well as in gamete function; and 3) Uncovering how a highly conserved RNA-binding protein, the Cytoplasmic Polyadenylation Element Binding Protein-2 (CPEB2) identifies and regulates mRNAs whose products are required for spermatogenesis and extrinsic regulation of sexual maturity. Due to their regenerative abilities and ease of systemic knockdown in planarians, the functions for hundreds of genes can be assessed during the award period. Completion of this project is important because it will reveal factors and mechanisms that will improve our understanding of the processes involved development of functional gametes. Additionally, this work will pave the way for future detailed mechanistic studies of specific germline genes in planarians as well as validation of orthologous gene function in mammalian systems. Altogether, drawing a more complete picture of the events involved in development of the germline is of outmost significance towards efforts in remediating infertility, developing novel contraceptives, and tackling pathologies of the reproductive system.